Method of strengthening alumina and spinel bodies

ABSTRACT

A METHOD OF STRENGTHENING ALUMINA AND SPINEL BODIES BY FORMING AT AN ELEVATED TEMPERATURE ON AT LEAST ONE SURFACE OF THE BODY A COATING OF A LOW EXPANSION MATERIAL THE METHOD COMPRISES THE STEPS OF PACKING THE CERAMIC   BODY IN A MIXTURE OF CR2O3 AND CRF3, CRCL3, ALF3, CAF2 OR NH1F. THE THUS PACKED CERAMIC BODY IS THEN REFIRED.

Sept. 11,1973 Y H. P. KIRCHNER ETAI- 3,758,323

METHOD oF STRENGTHDNING ALUMINA AND SPINEL BODIES Filed Aug. 25. 1970 4 sheets-sheet 1 aoss lrloaoaad mow-mamon Nooms 903@ Nzaoaad :mw-mamon wmwoaHo ATTORNEYS 4 Sheets-Sheet 2 H. P. KIRCHNER ET AL METHOD OF STRENGTHENING ALUMINA AND SPINEL BODIES OBW .LNBDBd 310W -lNLNOD WISBNOVW sept. 11,1973

Filled Aug. 25, 1970 foiv maagd Snow -lNBlNoQ wnmlwmv ENRY l RALPH WALKER BY WZ?? 2 g ATTORNEYS Sept. ll, 1973 HA P. KIRCHNER ET AL 3,758,328

METHOD OF STRENGTHENNG LUMIN ANDYSPTNEL BODIES 4 Sheet's-Sheet 3 Filed Aug. 25, 1970 o o@ o@ ov om om o 9 @m50/12 .I3 @mg o l| om mmm. 3Q. EQ. @5. o O o @E I. o mm @E o C @@Q. MPE. own.. Q O o, o@ @wmf Om Awww! 0 o OQQ. O @wmf o lm@ ATTORNEYS METHOD oF STREN'GTHENING ALUMINA AND SPINEL BODIES Filed Aug. 25, 1970 Sept. 11, 1973 H. P. KIRCHNER ET AL 4 Sheets-Sheet 4 S R m N E V m HENRY F3. KIRCHNER RALPH E. WALKER BY fwfdF- ATTORNEYS nited States Patent O 3,758,328 METHOD F STRENGTHENING ALUMJNA AND SPINEL BODIES Henry P. Kirchner, State College, and Ralph E. Walker,

Julian, Pa., assignor to Henry P. Kirchner, State College, Pa.

Continuation-impart of abandoned application Ser. No. 669,859, Sept. 22, 1967. This application Aug. 25, 1970, Ser. No. 66,785

Int. Cl. C23c 17/00 U.S. Cl. 117-107.2 P 11 Claims ABSTRACT OF THE DISCLOSURE A method of strengthening alumina and spinel bodies by forming at an elevated temperature on at least one `surface of the body a coating of a low expansion material. The method comprises the steps of packing the ceramic body in a mixture of Cr203 and CrF3, CrCl3, AlF3, Cal-72 or NH4F. The thus packed ceramic body is then retired.

This application is a continuation-in-part of application Ser. No. 669,859, filed Sept. 22, 1967, now abandoned.

The present invention relates to a method of physically strengthening polycrystalline ceramic bodies, specifically bodies of alumina and bodies of spinel, and to the alumina and spinel bodies strengthened by this method.

By the term spinel is meant the specific magnesium aluminum oxide material found in nature or the artificial material similar in composition and structure, but without most of the impurities.

Alumina bodies and spinel bodies are finding more and more uses in present day technology, Such as in radomes, windows for passing radiant energy in electronic tubes, and in structures used in aerospace technology. In addition, these ceramic materials have found considerable use in refractory technology.

A greater drawback in the successful use of such materials has been their relatively fragile nature, both with respect to ordinary physical stresses and strains as Well as so-called thermal shock. While these materials can withstand compressive stresses quite well, they are quite poor in their ability to withstand tensile stresses and bending stresses. This quite naturally limits their use in such structures as radomes and the like. This inability to withstand tensile stresses well also bears on their inability to withstand thermal shock. They withstand so-called temperature up shock, i.e. the stresses developed within the material when it is being heated up, quite Well, since the material tends to expand during the heating, thus becoming subjected to compressive stresses withinthe contines of the outside surfaces of the body. However, such materials do not withstand so-called temperature down shock, i.e. the stresses developed within the material when it is cooled. This is because when the contraction due to the cooling takes place, tensile stresses occur within the surface layers, and these tensile stresses are not well withstood by the material. As a result, flaws develop within the surface layer of the material which can in turn lead to catastrophic failure of the entire body of the material.

It is an object of the present invention to provide a method of treating alumina bodies and spinel bodies so as to improve their bending strength, thermal shock resistance properties, and hardness.

It is a further object of the invention to provide treated bodies of alumina and a spinel which have improved bending strength, thermal shock resistance and hardness characteristics.

It has been found that if there is formed at an elevated temperature on at least one surface of an alumina body rice or a spinel body a coating of a low expansion material, which in a preferred embodiment is a solid solution of a plurality of ceramic materials having a coefficient of expansion which is less than that of -the ceramic of the body, when the body is cooled, there is produced on the surface layer thereof a layer which is under compression. Preferably the coating is formed in a way such that an additive material which mixes with the material of the body to form the compressive coating and the material of the body itself are present in proportions which Vary from a high proportion of additive material to a low proportion of the additive material the deeper into the body the additive material penetrates. This results in a stress gradient in the finished material with the greatest compression of the coating being at the surface thereof, and the compressive stresses decreasing in the direction of the center of the body.

More particularly, this method comprises forming at an elevated temperature on at least one surface of an aluminum oxide body or a spinel body, a coating of a low expansion material which in a preferred embodiment, is a solid solution of metal oxide ceramic materials, one of which is a metal oxide of the body, and which solid solution has a lower coefficient of expansion lthan the coefficient of expansion of the body.

One particular method of forming the low expansion material coating has been to pack the body, which may or may not have been pretreated by fiuorinating it, in chromium oxide, and retiring the thus packed body to form a solid solution of chromium oxide and aluminum oxide. While these methods have been found to increase the bending strength and thermal shock resistance characteristics of alumina and spinel, we have now unexpectedly found that these characteristics can be still further increased by including a fluoride or chloride in the chromium oxide.

The method of the present invention comprises packing the alumina or spinel body in chromium oxide, either with or without a iiuorinating pretreatment, which chromium oxide includes a fluoride or chloride taken from the group consisting of Cri-i3, CrCl3, All-T3, CaF2 and NH4F, and then retiring the body. It has been found that a low expansion coating of a solid solution is formed which greatly increases the strength of the body as compared with the method in which there is no iiuoride or chloride included in the packing material.

While in the following examples the retiring of the ceramic bodies has been carried out at temperatures in the vicinity of from 1500 C. to 1725 C., it is contemplated that the firing can be carried out anywhere between 1000 C. and a temperature below the melting point of the ceramic material being fired where no pressure is applied to the bodies during tiring, and depending on the nature of the materials and the degree of reaction between the raw materials of the coating and the body.

The invention will now be described in greater detail with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of apceramic body having a low expansion coating thereon in accordance with the present invention;

FIG. 2a is a view of the aluminum, chromium profiles of the composition of the surface layer of a body packed only in Cr203 and fired;

FIG. 2b is a silicon profile of the composition of the surface layer of a body which has been leached and then packed in Cr203 and a fluoride and fired;

FIGS. 3a and 3b are views of the aluminum and magnesium profiles of bodies treated the same as the bodies of FIGS. 2a and 2b;

FIG. 4 is a graph of flexural strength v. percent of fluoride for additions of various amounts of CrF3 and NH4F to the packing material; and

FIGS. Sa-Sc are microphotographs of the surface texture of alumina packed in Various amounts of Cr2O3 and CIFg. l

As seen in FIG. 1, a body 1 of ceramic material 2, for example aluminum oxide or spinel, has at least one surface, the top surface in this instance, coated with a low expansion solid solution 2 of a plurality of ceramic materials. The coating materials and the ceramic of the body are intermixed at the interfacing of the coating and the material of the body, there being a gradual increase in the proportion of alumina and a gradual decrease of the proportion of coating material the deeper into the body the determination of the relative amounts of these materials is made. The low expansion coating 3 is of a material which has a lower coefficient of expansion than the ceramic material 2 and is under compression as a result of having been formed on the material 2 at a high temperature, and the temperature of the overall structure reduced to ambient temperature. Since the relative amount of coating material 3 decreases as the distance from the coated surface increases, there is also a stress gradient, the compressive stress being greatest at the surface of the coating and decreasing in proportion to the amount of coating material the greater the distance from said the same conditions, the bodies packed in the fluoride containing chromium oxide have significantly greater flexural strengths.

In order to further illustrate the method of the present invention and the results obtained thereby, there will be given a series of specic examples.

EXAMPLES l-6 94% alumina bodies purchased from Frenchtown Porcelain Company (No. 7225) in the form of tiles 3 x 3 x 1A" Were cut into test bars 3 x 1A x 9/64 by diamond sawing.

A iirst group of 5 samples was used as controls, and a second group of 5 samples was packed in Cr203 only and retired at 1500 C. in a gas kiln for four hours. Six further groups of tive samples each were then packed in various mixtures of Cr203 and CrF3-31/2H2O, except for the last group in which the Cr2O3 was omitted. The samples were retired at the same conditions as the second group of samples. The proportions of Cr203 and CrF331/2H2O, and the type of kiln used to retire the respective groups are given in Table I, together with the average flexural strengths of the retired bodies.

TABLE 1.-FLEXURAL STRENGTI OF ALUMINA PACKED IN VARIOUS MIXTURES OF m03 AND GIFs-3%H20 [Frenchtown 94% alumina retired at 1,500 C. forfour hours) samples cut from tiles] Average Strength tiexural difter- Num- Number strength, ence., ber Treatment samples p.s.i. p.s.i.

5..- Packed in 01'203 plus 70% CrF3-3%H2O, electric klln. 5 50, 800 +16, 600 5 41, 700 +7, 500

6 Packed in 100% CrF3-31/QH2O, gas kiln coated surface. In practice it has been found that the good results are achieved when the coating material penetrates into the material 2 of the body a distance of from about .001 on relatively thin bodies, and up to 20% of the thickness of relatively thick bodies. It should be understood that in the present invention the size of the body is increased only slightly. The great majority of the coating material penetrates the material ot the body.

In order to produce a body as shown in FIG. 1, a body of alumina or spinel is prepared. It may, if desired, first be fluorinated, for example by soaking it in hydrofluoric acid or subjecting it to the action of hydrogen uoride, for suflicient time to at least leach out surface impurities, after which it is packed in a mixture of chromium oxide, Cr203, and a fluoride, for example chromium fluoride CrF3-31/2H2O. The preferred ratio of chromium oxide to fluoride is from 20-7 0% fluoride. Thereafter, the packed body is retired at a temperature of from 1000 C. to a temperature just below the melting point, but preferably from about l500-1725 C. for a period of from 1-4 hours.

Tests to determine the exural strengths of the bodies shows that as compared to the as received controls and bodies packed only in chromium oxide and retired under It will be seen that except for Example 1 where the proportion of fluoride is low, and Example 6 Where there is no Cr2O3 present, the exural strengths are higher than when Cr203 is used alone.

EXAMPLES 7-10 Pretreatment by tluorinating the alumina bodies is effective to increase the strength of the bodies. In these examples, six further groups of alumina bodies having the same characteristics as those in Examples 1-6 Were prepared and each group was leached in hydrofluoric acid to a depth of 0.010 in., and after rinsing was treated as listed in Table II. The samples of Example 7 were dipped four times in CrCl3 solution, and then packed in Cr2O3 and retired at 1650 C. for one hour. The samples of Example 8 were packed in Cr2-03 With 20% CrF39H2O While the samples of Example 9 were packed in Cr203 with 40% CrF331/2H20. The samples of Example 10 were packed in Cr2O3 with 20% AIF3'6H2O, while the samples of Examples 11 and 12 were packed in Cr203 and 10% CaF2 and CrCl3-6H2O, respectively. All were retired at the same conditions as the controls and the samples of Example 7. The proportions ot the packing materialsare given in Table II as are the average flexural strengths of the red bodies.

TABLE II.-FLEXURAL STRENGTH OF ALUMINA PACKED IN MIXTURES OF CrzOa PLUS VARIOUS FLUO RIDES AND CHLO RIDES [(Frenchtown 94% alumina tired at 1,650o C. for one hour) samples cut from tiles] Average Strength flexural differ- Num- Number strength, ence, ber Treatment samples p.s.i. p.s.i.

Controls, retired 30 30, 900

Controls, leached to 0.010 in., retired 4 29, 900

7 Leached to 0.010 in., dipped 4X in C1C13 soln,. packed in Cr203 30 36, 100 +6, 200 8 Leached to 0.010 in., packed in 80% CrzOa plus 20% Crm-95120...- 5 45,300 +15, 400 9..- Leached to 0.010 in., packed in 60% CrzOa plus 40% CrFa'l/HzO-- 19 46, 400 +16, 500 Leached to 0.010111., packed in 01'203 plus 20% AlF3-6H2O 5 38,800 +8, 900

Leached to 0.010 in., packed in of CrzO; plus 10% CaFr 4 39, 700 +9, 800

Leaehed to 0.010 in., packed in 90% CrzOa plus 10% CrCl3- H2O... 5 41, 300 +11, 400

Improvements in strength were observed for additions of chromium fluoride, aluminum liuoride, and chromium chloride to the Cr203 packing material Chromium 11u0- ride additions led to greater penetration of the chromium into the body, and improve surface texture. Additions of calcium fluoride also resulted in an increase in strength.

When Cr203 alone is used as the packing material for unleached alumina, silicon accumulates in the surface layer, as shown in FIG. 2a. Leaching and packing in Cr2O3+CrF3 3%/2H2O prevent this accumulation of silicon as shown in FIG. 2b.

Packing in material containing the fluoride results in accumulation of magnesium in the surface layer as indicated by comparing FIGS. 3a and 3b. The aluminum content of the surface layer is very low in the region where the magnesium accumulates so that the magnesium is probably present in the form of picrochromite (magnesium chromium spinel, MgO-Cr203). Since picrochromite has a lower expansion coetlicient than Cr203, the presence of this phase in the surface layer aids in 0btaining high compressive stresses.

EXAMPLE 13 A group of samples corresponding to the samples of Examples 7-12 was prepared, and the samples were leached in hydrotluoric acid to a depth of 0.010 in. by soaking in hydrotluoric acid. The samples were then packed in Cr203 without being rinsed and were then retired at 1650 C. for one hour. As compared to the retired controls of Table II, the average flexural strength was 46,400 p.s.i., a strength ditterence of 446,500 p.s.i.

TABLE IIL-EFFECT. OF FIRING CONDITIONS ON FLEXURAL STRENGTH OF TREATED ALUMINA (Frenchtown 94% alumina packed in 50% CrzOa plus 50% CrFayHzO) Firing Average conditions tlexura tempera- Time, Number strength ture, C. hours samples p.s.i.

1 Samples leached to depth of 0.010 in.

EXAMPLE 19 The invention is also applicable to spinels. Four groups of Degussa SP-23 Spinel samples were prepared in the form of hollow cylinders, the first group of which were used as controls, the second group being simply retired at 1650" C. for one hour, and the third group being packed only in Cr203 and retired in a like manner. The samples of Example 19 were surface fluorinated with hydrouoric acid and were packed in a mixture of 90% Cr203 and 10% CrCl3'6H2O, and were then retired under the same conditions as for the other spinel bodies. The tlexural strengths were as shown in Table IV, from which it is seen that the strength is increased when the chloride is added to the chromium oxide in which the bodies are packed for retiring.

TABLE IV.-FLEXURAL STRENGTH 0F DE GUSSA SP-23 SPINEL WITH VARIO US PACKING MATE RIALS Flexural strength data Firing Conditions Average flexural Strength Temp., Time, Number strength, dilerence, Number Treatment C. hours samples p.s.i. p.s.i.

Degussa SP-23 controls, as cut 4 21,850 Degussa SIP-23 controls, rened 1, 650 1 5 20, 400 Degussa SP-23 packed in CrzO; 1, 650 1 5 25,300 4, 900 19 Degussa SP-23 surf. uor., packed in 1, 650 1 5 26, 550 6, 150

EXAMPLES 14-18 and the resulting samples had average exural strengths 55 as shown in the table.

EXAMPLES 20-35 A group of 5 alumina rods 0.138" in diameter and made of 96% alumina Al Si `Mag 614, American Lava Corporation) were packed in Cr203 and tired for 1 hour at 1500o C. A set ofv eight groups ot 5 rods each were packed in mixtures of Cr2O3 and CrF3-31/2H2O in various weight ratios, and these eight groups of rods were also tired at 1500 C. for 1 hour. A further set of eight groups of 5 rods each were packed in mixtures of Cr203 and NH4F in various weight ratios and also tired at 1500 C. for 1 hour. The various weight ratios were as set forth in Table V attached hereto. The resulting specimens were US FLUORIDE MIXTURES (0.138" diam. rods, 1500 C., 1 hour) Average exural Average Number strength, diameter, Number Treatment samples p.s.i. in.

20 Packed in CrgO; 5 51,500 .1396 21 Paked in 90% CrgOa plus 10% CrF3.3/ 5 52, 100 1386 tested for flexural strength by 4 point loading on a 2 inch span. The average diameters of the specimens and the average ilexural strengths were found to be as shown in attached Table V, and the average flexural strengths are plotted in FIG. 4. Diameters are shown next to the plotted strengths. l

As can be seen from the data in Table V and from FIG. 4, the strength of the specimens is increased significantly as compared with the specimens packed in Cr2O3 only, particularly where the amount of the halide added is 20% and greater. The graph shows that the strength of the specimens where the NH4F is added to the packing reaches a peak sooner than where the CrS3 is added.

Reference is made to the diameters of the rods as evidence that more is taking place than a mere transfer of the packing material to the surface of the rods. The diameters increase only slightly when the method according to the present invention is carried out. It is therefore clear that what is happening is that the transferred material is penetrating into the surface layer and combining with the material of the body to form the low coeicient of expansion layer which is contributing to the increased strength being sought.

EXAMPLES 3 6-3 8 A group of 5 alumina rods 0.138 in diameter and made of 96% alumina (Al Si Mag 6.14, American Lava Corporation) were packed in Cr2O3 and tired at 1500 C. for 1 hour. A second group of 5 identical rods were packed in a mixture of Cr2O3 and 0.5% by weight NH4F and fired under the same conditions, while a third group of 5 identical rods were packed in a mixture of Cr2O3 and 0.5% by weight NH4F and 0.1% by weight NH4NO3 and also fired at the same conditions. The resulting specimens were tested for flexural strength by 4 point loading on a 2 inch span. The average diameters of the specimens and the average flexural strengths were found to be as shown in Table VI, which also gives the same data for NH4F, and retiring the thus packed ceramic body for a time and at a temperature for diffusing the CrO3 from the packing to the body and diffusing it into the body and combining it with the material of the body to form said low coefficient of expansion material, and for forming a surface on said strengthened body having an improved surface texture as compared to a body formed by firing Without the halogen containing salt.

2. A method as claimed in claim 1 in which, when the halogen containing salt is CrF3 or AlF3 it is present in the mixture in an amount of at least by weight, and when the halogen containing salt is CrC13, CaF2 or NH4F it is present in the mixture in an amount of at least 10% by weight.

3. The method as claimed in claim 1 in which the mixture of materials in which the body is packed is 80% Cr2O3 and 20% AlF3-6H2O.

4. The method as claimed in claim 1 in which the mixture of materials in which the body is packed is a mixture of Cr2O3 and CrFa.

5. The method as claimed in claim 4 in which the amount of CrF3 is from 20-70% of the mixture.

6. The method as claimed in claim 5 in which the fluoride is in crystalline form.

7. The method as caimed in claim 5 in which the rering temperature is from 15001725 C.

8. The method as claimed in claim 1 in which the mixture of materials in which the body is packed is 90% CrO3 and 10% CrCl36I-I2O.

9. The method as claimed in claim 1 in which the mixture of materials in which the body is packed is 90% Cr2O3 and 10% CaF3.

10. The method as claimed in claim 1 in which the mixture of materials in which the body is packed is 90% Cr203 and 10-70% NH4F.

11. A method of strengthening a body of a ceramic material taken from the group consisting of alumina and spinel of the Al2O3-Mg0 type by forming at an elevated TABLE VI.FLEXURAL STRENGTH OF 96% ALUMINA PACKED IN VARIO US FLUO RIDE MIXTURES Treatment conditions Average flexural Temp., Average Number strength, Number Treatment C. Time diameter samples p.s.i.

Refired controls... 1, 500 1 1372 5 59. 600 36 Packed in Cr2O3 1. 500 1 .1394 5 53,000 37...- Packed in CrzOa plus 0.5% NH4F 1,500 1 1406 5 49,100 38 Packed in CHO; plus 0.5% NH4F plus 1, 500 1 1416 5 53, 300

0.1% NHtNOs.

a group of identical rods which were simply fired along temperature on at least one surface of the body a coating with the above described Specimens, of a low coefiicient of expansion material having a lower It is thus seen that the use of the -very small amount of ammonium fluoride in the packing does not increase the strength while it increases the size of the specimens. In the present method, on the other hand, the strength of the specimens is increased without significantly increasing the size of the specimens.

The method of the present invention improves the surface texture of the bodies. FIGS. Saz-5c are three micr0- photographs of the surface texture of alumina packed in Cr2O3 only, and in various mixtures according to the invention, and then retired at 1650 C. for 1 hour. The improved surface texture is most noticeable for the 50% Cr2O3-50%I CrF3-31/2H2O mixture. The fine texture of the surface contributes to the improved strength by reducing the magnitude of the surface flaws.

What is claimed is:

1. A method of strengthening a body of a ceramic material taken from the group consisting of alumina and spinel of the AlZOS-MgO type by forming at an elevated temperature on at least one surface of the body a coating of a low coeicient of expansion material having a lower coefficient of expansion than said ceramic material, said method comprising the steps of packing the ceramic body in a mixture of CrzOa and a halogen containing salt taken from the group consisting of CrF3, CrCl3, AlF3, CaF2. and

coelicient of expansion than said ceramic material, said method comprising the steps of soaking the body in hydrofluoric acid for leaching the surface thereof, packing the ceramic body in a mixture of Cr203 and a halogen containing salt taken from the group consisting of CrF3, CrCl3, A1F3, CaF2 and NH4F, and rering the thus packed ceramic body for a time and at a temperature for diffusing the Cr203 from the packing to the body and diffusing it into the body and combining it with the material of the body to form said low coeicient of expansion material, and for forming a surface on said strengthened body having an improved surface texture as compared to a body formed by firing without the preliminary soaking.

References Cited UNITED STATES PATENTS 4/ 1947 Baumann et al 106-66 X 4/ 1968 Weatherley 117-107.2 P

ALFRED L. LEAVITT, Primary Examiner D. A. SIMMONS, Assistant Examiner 

